The nervous system of the giant barnacle provides a relatively simple and approachable preparation in which to study the transfer of information in a visual pathway. Only four photoreceptors comprise the median eye; they project to the supraesophageal ganglion where the visual information is processed by a small number of ganglion cells. We have been studying the properties of these photoreceptors and of second-and third-order cells in the visual pathway. We have been able to propose a scheme that can largely explain how a depolarizing, sustained photoreceptor response to light is converted to a transient, amplified response when the light is turned off ("off-response") in higher-order cells. The second-order cells emerge from this study as neurons with particularly interesting properties. With intracellular recording techniques we will study the membrane properties of these second-order cells and how these properties generate the cell's response to light. We will be particularly interested in the response of the second-order cell during background, adapting lights and also how the receptor terminals function under these conditions. We will attempt to identify the transmitter released by the photoreceptor with biochemical as well as physiological techniques. Knowledge of this compound would give us a powerful tool for studying further the release and reception of transmitter at this synapse between two decrementally-conducting cells. Integration of information from the medial eye with that from the two lateral eyes will be studied by recording from lateral receptors and from higher-order neurons. Since each lateral eye contains only three receptors, we expect to be able to describe this visual system with an unusual degree of completeness. The similarity of certain anatomical and physiological features of this system to those of the vertebrate retina make it of particular interest for studying the membrane and synaptic properties that underly visual integration.